The present invention relates, in general, to a valve driving device, and more particularly to a tubular cam which is used to drive a follower connected to a valve such as the intake valve or the exhaust valve of a four-cycle engine.
As is well known in the prior art, engine valves are conventionally driven by a cam shaft through a rocker arm assembly, with valve springs surrounding the valve stems to drive the valves to their closed position. The tips of the air intake and exhaust valve stems are contacted by corresponding pivoting rocker arms so that the valves are driven open by the movement of the rocker arms, and are returned to their closed position by their corresponding valve springs. When engines utilizing such a construction are operated at high speed, however, the valve timing is often adversely affected so that the valve closing is delayed and the valve remains open at times in the cycle when it should be closed. This is caused by the fact that the spring cannot operate quickly enough to close the valve, and at very high speeds this sometimes results in valve flotation.
One correction for the foregoing problem is the provision of a stronger spring. However, when such a spring is used, it becomes necessary to use considerable force to open the valve, and this pressure on the spring produces tremendous friction on the various contacting parts, which produces additional problems.